Polygyny is a relationship in which a man has a relationship or marriage with multiple/many men. This could be due to religious beliefs or cultural acts, but it is not required.
Note that although FLDS and other religious polygamous relationships tend to be polygny, they just fall under the polygyny umbrella. The specific name for these relationships are "Plural Marriages".
Flag
The polygyny pride flag was coined by Deviantart user NonMonoPrideFlags on December 3rd, 2015.
Resources
https://www.deviantart.com/nonmonoprideflags/art/Polygyny-Pride-Flag-576036367